Direct conversion receivers convert signals directly to baseband for subsequent processing and thus eliminate the use of converters and intermediate filters typical of other receivers, for example those of superheterodyne receivers. Intermediate frequency receivers, for example very low intermediate frequency (VLIF) receivers, convert signals to an intermediate frequency for processing before converting to baseband.
Direct conversion receivers are less complex and less costly relative to intermediate frequency receivers, and thus there is a strong desire to implement direct conversion receivers in communications devices, for example in mobile cellular communications handsets.
The Universal Mobile Telecommunications Service (UMTS) wide band code division multiple access (WCDMA) communication standard has frequency spectrum allocated exclusively to WCDMA communications. The exclusively allocated WCDMA frequency spectrum is substantially devoid of narrow band blockers. Other non-exclusively allocated frequency bands used for WCDMA communications, however, are subject to high-level narrowband blockers, which produce 2nd and higher order intermodulation distortion products, which degrade receiver performance.
The distortion may be mitigated in direct conversion receivers with a high 2nd order intercept mixer, as described for example in U.S. Pat. No. 6,192,225 entitled “Direct Conversion Receiver” and in U.S. Pat. No. 6,125,272 entitled “Method And Apparatus Improved Intermodulation Distortion Protection”, and by baseband frequency filtering, but these techniques are often insufficient for many applications.
Distortion may be reduced in intermediate frequency receivers by filtering the signal before conversion to baseband. Filtering before conversion to baseband however works well only for applications where the baseband sampling rate is sufficient to accommodate the intermediate frequency offset and where adjacent channel rejection requirements are less severe.
The various aspects, features and advantages of the present invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description of the Invention with the accompanying drawings described below.